Ferroelectric materials are used in microwave devices to control the propagation of a microwave signal. This use in microwave phase shifters and quasi-optical antenna arrays is described in Varadan et al. (Microwave Journal 34, 116 (1992)), Babbitt et al. (Microwave Journal 35, 63 (1992)), and Vendik & Ter-Martirosyan (Microwaves & RF, July, 67 (1994)).
In a ferroelectric phase shifter, for example, the microwave signal is loaded with ferroelectric material in a way such that the microwave signal must interact with (or travel through) the ferroelectric. The geometry must also allow for the application of a dc bias voltage across the ferroelectric material.
A typical ferroelectric material for this use is barium strontium titanate (hereinafter referred to as BST). It is known that when a dc electric field is applied to BST, the dielectric constant of the BST decreases with increasing field strength.
Thus, in a ferroelectric phase shifter, when a dc electric field is applied across the ferroelectric material, i.e. BST, the change in dielectric constant changes the effective electrical path length of the waveguide and therefore, the output signal changes phase relative to the zero dc bias condition. Thus, the change in dielectric constant due to an applied dc electric field is the fundamental basis of operation for any ferroelectric microwave device.
The operating characteristics of current ferroelectric microwave devices, such as phase shifters, are strongly affected by temperature because the electrical properties of the ferroelectric material change rapidly with temperature. In particular, it is known that for many ferroelectric materials, and for BST in particular, the large grain size of the material influences the electrical properties.
The prior art reveals an important problem associated with the current generation of ferroelectric devices. A small change in temperature of the device will result in a change in the dielectric constant even without an electric bias. Any device using this type of material will be, therefore, highly temperature sensitive.
It is an object of the present invention to provide a method which eliminates or greatly reduces the temperature sensitivity of ferroelectric material in a ferroelectric microwave device.